1. Field of the Invention
The present invention relates to thermionic cathodes, in particular for use in a lithography system, in particular a multi-beam lithography system.
2. Prior Art
Thermionic cathodes, more specific controlled porosity dispenser-type thermionic cathodes, are well known in the art. These dispenser cathodes are commonly used in for instance televisions, computer monitors and microwave ovens. Such a dispenser cathode usually comprises a cathode body with a reservoir comprising a cavity filled with work function lowering material and with an emission surface, and a heating element for generating the heat needed to cause work function lowering particles to diffuse from the reservoir to an emission area on the emission surface and to create the thermionic emission. In general, the entire emission surface is used as emission area.
There are several types of reservoirs known in the art. In a first type of reservoir, the cavity is filled with a porous matrix and the pores of this matrix are filled with the work-function-lowering particles, e.g. compounds of alkaline earth metals like Ba, Ca and Sr. In these dispenser cathodes, usually one surface of the porous matrix functions as emission surface. Heating the cathode will release work function lowering particles from the porous matrix to the emission surface and will cause thermionic emission.
In a second type of reservoir, work function lowering material is present in a space behind the porous matrix. During operation work function lowering particles, e.g. Bα or BαO, are generated or released within the pores and are supplied from the space behind the porous matrix and then migrate through the pores of the porous matrix and are supplied to the emission surface in sufficient quantities to maintain good emission surface coverage which assures adequate emission from the emission surface.
In order to increase brightness of these type of cathodes, a coating is deposited on top of the porous matrix, usually on the entire surface of the metal matrix. Thus, the entire emission surface becomes an emission area. The coating usually has several layers, at least one of which comprising a work-function-lowering material. Nowadays the most widely used work-function-lowering material for this coating layer comprises a scandate compound. Appropriate fabrication techniques and components of these scandate dispenser cathodes are for instance disclosed in U.S. Pat. Nos. 4,007,393, 4,350,920, 4,594,220, 5,064,397, 5,261,845, 5,264,757, 5,314,364, 6,407,633 and 6,348,756, which documents are all incorporated by reference as if fully set forth.
In these known cathodes, the connections between the pores are randomly generated. An inherent consequence is that the path length that the active materials must travel to reach the emission surface can be much larger than the thickness of the matrix layer. This limits the lifetime and emission of these conventional dispenser cathodes. Furthermore, these pores debouche in the emission surface, which forms a relatively large emission area in its entirety.
U.S. Pat. No. 4,101,800 discloses a controlled porosity dispenser cathode provided with a thin perforated foil on top of the matrix layer. The foil is made of a refractory metal. The active materials migrate through the holes in the perforated foil to coat the surface of the foil. The foil thus serves as the emission surface of the cathode. In this cathode again, the entire emission surface is emission area
A further improvement of this concept is disclosed in U.S. Pat. No. 4,310,603. The dispenser cathode disclosed in this document comprises a cathode body comprising a reservoir with work function lowering material and a heating element located in the cathode body on one side of the reservoir. The opposite side of the reservoir defines an emission side surface. The emission side surface of the reservoir is provided with a foil with holes covering the reservoir, welded to the cathode body and preferably made of tungsten or molybdenum. Parts of the foil are provided with a coating comprising work function lowering material defining emission areas, and parts of the foil are provided with non-emitting material defining a shadow grid. The work function lowering coating establishes a lower work function φ and thus an enhanced emissivity of the emission areas. In this cathode, all the holes are located in the large emission areas.
In these controlled-porosity dispenser cathodes described above, the dimensions of the pitch between the holes define the path length of the active materials. However, the holes in the emission surface also induce a severe inhomogenity in the radiation from the cathode. Furthermore, almost the entire emission surface acts as emission area.
The invention further relates to the use of a dispenser-type cathode in electron beam exposure apparatus like lithography systems, electron microscopes, inspection systems. In these electron beam apparatus generally a LαB6-source, a field emitter or a Schotky type emitter is used. Most of these apparatus a relatively low current is required, as these apparatus require a homogeneous electron source with a relatively small dimension. The use of a dispenser cathode with a high brightness in an electron beam apparatus is therefore not trivial. The diameter of a prior-art dispenser cathode, for instance, is 100-10,000 times larger than the diameter of a LαB6-source or field emitter source. Thus, the emission current is too high for the electron beam apparatus. Furthermore, since more electrons are emitted in the same period of time. Coulomb interactions reduce the resolution of an apparatus when using the known dispenser cathodes.
This problem is especially relevant in single source multi-electron beam systems. In an attempt to overcome this problem, especially in single source multi-electron beam systems, a diverging electron beam with a limited emission area is used. Examples of such systems are disclosed in for example U.S. Pat. Nos. 5,834,783, 5,905,257 and 5,981,954 by Canon and U.S. provisional application 60/422,758 by the present applicant, which is incorporated by reference as if fully set forth. A so-called triode setup is most often used to accomplish the diverging electron beam. The electric field lines produced by two electrodes, oppositely charged and in close proximity of the emissive cathode surface, create an expanding electron beam using only a fraction of the emitted electrons. However, before the electrons diverge, their individual trajectories first converge and go through a crossover. This results in stronger Coulomb interactions and consequently in an increase of the energy spread. This extraction approach thus poses a problem, especially in high-resolution systems, like electron beam lithography systems.